Presently, steel scrap is taken from major industrial areas, shipped to remote sites, melted in arc furnaces, either refined in the arc furnace or a ladle metallurgy station and then direct cast into ingots, billets or slabs. The ingot, billets or slabs are then shaped into various end products by conventional rolling or casting means. Additionally, work is being done to develop casters which will continuously cast such end products as rods, wires, sheets and slabs.
Heretofore, the steel industry has used ladle "arc" refining technology as a method for optimizing and increasing the flexibility of the steel production process. The state of ladle "arc" refining furnace technology is set forth in an article by R. J. Fruehan, Professor and Director of CISR, Carnegie-Mellon University, entitled "Ladle Refining Furnaces for the Steel Industry", a report prepared for the Center of Metals Production, in March 1985. In the article, ladle furnaces are described as "not unlike electric arc furnaces", Supra, page 2. Furthermore, the report notes that ladle "arc" furnaces typically range in size from 30-60 tons (a small unit) to 180-240 tons (a large unit). Supra, page 2. The report also notes that it is known to use induction stirring techniques in such ladle "arc" refining furnaces. Supra, Table 4.1.
Induction melting furnaces have been used by the steel foundry industry for approximately 20 years. For example, see the article by R. Duncan entitled "Coreless Induction Melting in Steel Foundries" written for Castell Technical Service of Lopez, Wash. But their application has been limited because at production rates of 100 tons per hour or higher, the furnaces have not proven successful because of their use of 60 cycle current induction technology.
Conventional steel making practice relies on the oxygen decarburization reaction to accomplish refining of the steel scrap or reduce iron used as the charge materials in the melt operation. The decarburization process is characterized by the addition of sufficient carbon during melt or subsequent to melting and then removing the carbon by use of either solid oxygen in the form of scale or ore, or by gaseous injection through lances or other means to achieve a rapid and violent boil. The benefits of carbon are first to lower the melting point of the solid mass thereby reducing the energy to form a molten pool, and second to refine by the generation of carbon monoxide which bubbles through the liquid steel causing the boil. Because most melting processes are relatively slow and usually carried out in air, such as electric arc melting, line frequency or 60 cycle coreless induction, oxidation of the raw materials in the furnace before they turn into liquid is quite prevalent and is another reason for the requirement of the carbon oxygen decarburization reaction.
Another reason for the need of a carbon boil is the increase in the quantities of hydrogen and nitrogen to the liquid steel caused by atmospheric conditions during a slow melting or the action of the electric arc and electric arc melting which ionizes these gases that are then absorbed by the melt. The decarburization reaction flushes excesses of these gases from the melt to tolerable levels.
The action of the arc in the electric arc melting of steel, the slowness and unfavorable melting characteristics of line or 60 cycle frequency coreless induction furnces, and the lack of sufficient power capabilities of the early and non-solid state medium frequency coreless induction furnces made it necessary to rely on the carbon oxygen reaction to form gas bubbles for refining steel prior to the instant invention.
Prior to the instant invention, steel production at micro mill rates, disclosed hereinafter, were either not capable of being produced economically or else required processing methods such as the carburization/decarburization action in order to be economically viable.